There are many known systems and devices intended to protect jet engines in general, and those on aircraft in specific. Nearly all employ some type of shield in front of the inlet, and they have taken various forms and use a variety of means. As in many fields of endeavor, successive developments are made but subsequently are found lacking as new understandings and problems arise out of often unintended consequences. A review of patents directed to such systems and devices confirms this, and also shows that one specific issue dominates. One central problem in this field involves failed attempts to avoid a significant reduction in airflow that arises from the specific system or device being made. Several such designs claim to have solved this problem, but each of the proposed solutions brought with it some further complication, e.g., if a retractable shield was presented as the solution, the shield and related mechanisms became prohibitively complex and heavy.
Also, it is apparent that several of these prior attempted solutions were based on premises that either no longer apply or were flawed. One such assumption, used by certain designs, is that bird strikes occur only in that portion of the flight at or when approaching an airport. While it is true that a large percentage of recent bird strikes have occurred there, a system designed to operate only in this flight region leaves the aircraft vulnerable to bird strikes that can and do occur in all phases of flight, including at high altitude. Publicly available data show damage to aircraft from routine encounters with birds at flight altitudes of 10,000 feet and well above.
None of these known systems has found its way into practical application. Also, nearly all are limited in application because they have potential use only for an older type of aircraft jet engine—the turbojet. Few relate to the situation and circumstances presented by the newer engine design now widely in use on commercial aircraft—the high-bypass ratio turbofan—and those few contain significant problems yet to be successfully addressed.
Since the time of the earliest patents in this field, inventors have envisioned affixing some type of metal screening structure in front of the engine intake for the general purpose of preventing the ingestion of debris. However, the preponderance of patents specifying this type of screen device recognizes that it causes problems. For example, U.S. Pat. No. 2,507,018, dating from 1950—only just over 10 years after the first jet engine flew—was an annular inlet screen, inside the inlet duct, sloping forward from nacelle cowling to center hub, and made of aerodynamically shaped metal slats attached to aerodynamically shaped struts. This entire apparatus was to be electrically heated in an attempt to deal with ice buildup on the struts. Several patents followed that provided alterations on the type and/or placement of the metal screen system, and all dealt with the need for anti-icing.
To operate as designed, a jet engine needs to have a large quantity of air flowing into the inlet, with the flow essentially undisturbed by any object encountered prior to entering the inlet. A problem not addressed by some early designs and that they created or contributed to was a significant reduction in airflow caused by the blockage of the inlet by the metal system. Later designs attempted various techniques to address this problem. For example, U.S. Pat. Nos. 3,196,598; 3,871,844; 4,149,689; and 5,411,224 each address this problem with techniques such as placing the screen in front of the inlet and making it a very large oval- or cone-shaped apparatus when viewed from the side, and shaping the screen material into a sort of airfoil cross-section, with the intent of reducing inlet blockage.
Other designs proposed using a screen having movable members, allowing the screen to be in a sort of retracted mode for what was considered non-hazardous portions of the flight. However, such designs presented several problems, including very complex mechanisms needed for movement between their stored and operational states. Also, although birds are more likely to be encountered by aircraft during the lower altitude flight departing or arriving at an airport, there have been many damaging bird strikes in other flight regimes, and those designs do not provide aircraft engine protection throughout the entire flight envelope.
Three recent patent publications mention the modern turbofan jet engine or show a configuration on what is possibly a variant of a modern jet engine. U.S. Pat. No. 6,089,824 proposes a solution that instead of placing a screen across the inlet to prevent bird ingestion; a cone-shaped, spinning cutter is attached to the rotating engine shaft out in front of the inlet, and is intended to dismember incoming birds. U.S. Pat. No. 6,138,950 describes a concept resembling that originally employed in the 1970s on one military aircraft—the Lockheed F-117A “Stealth Fighter”. In this implementation the inlet is covered with a thick plate that forms a porous grid including a set of adjoining tubes. Several different aircraft are shown with the device, including one installation on what resembles a turbofan engine. International patent publication WO/2001/012506 describes a device that includes elements which can move between a first, inactive position in which the air inlet is substantially open; and a second, active position in which the elements form the protection in front of the inlet opening.
Also, U.S. Pat. No. 7,494,522, corresponding to International patent publication WO/2007/0245697 describes numerous designs, all of which involve screens that are mostly inside the engine itself and attached to a rotating pole.
Bird strikes are becoming more frequent, though mainly they have not caused catastrophic loss of aircraft and lives. The recent loss of a commercial aircraft to bird ingestion—although amazingly all the passengers survived—has brought into sharp focus the reality of the massive increase in bird populations around the world, and especially in the continental United States. Experts in the field have stated prior to the accident referred to, that they had expected to lose aircraft to bird strikes. What they did not expect was that anyone would survive—much less the entire complement of occupants. Predictions are that bird populations will continue to increase. Along with this has been an increase in the mass (weight) of the birds and thus a higher percentage of such birds being involved in bird strikes. All modern jet engines are designed to U.S. Federal Aviation Agency-mandated requirements that specify the type of continued operation after ingesting a bird of specified size, at a specified aircraft speed. Modern jet engines have demonstrated the ability to achieve these specified requirements in test environments. However, a real problem exists in that in actual, non-test conditions birds that collide with jet engines have at times weighed more than what the FAA-mandated weight requirement specifies. While jet engine manufacturers have been able to occasionally demonstrate in tests successful handling of a collision with a somewhat heavier bird than the FAA requirement, such tests have not been consistently successful. As a result, large, heavy birds currently pose an unmanageable threat to commercial aviation safety.
Finally, turning to the modern turbofan jet engine, it is distinguished by its very large inlet, far beyond the dimensions of the turbojet engine, for which nearly all of the earlier jet engine protection designs were directed. The very size of this inlet challenges, if not renders useless virtually all of these previously known engine protection designs. Of those few that were intended for possible use on a turbofan jet engine, the inventors expressed concern about the crucial need to avoid reducing engine performance. While attempts were made to satisfy this objective, e.g. by having the engine protection system remaining completely imbedded within existing structure until ordered into play by some pilot action, none was designed to be completely autonomous. Also, none was designed to operate in fractions of a second, none was able to determine the need for actuation based on accurately assessing the mass of the approaching bird, none employed bird-shielding, deflecting, or destructive mechanisms and none addressed protection of the central region of the engine inlet, rather than the entire fan area, to prevent the ingested bird from travelling into the core of the engine. It is the engine core that is susceptible to complete and unrecoverable engine failure from such ingestion. All these problems and concerns are addressed in the systems and methods described herein.